1. Technical Field
This invention generally relates to filters, and more specifically to a spectrally flat time-domain equalizer (TEQ) filter, which increases the data rate of a communications system.
2. Background Art and Technical Problems
In modern electronic circuits, many different kinds of filters are used in a variety of applications. Filters are typically used to remove one or more components of a signal so that a xe2x80x9ccleanxe2x80x9d signal is obtained. For example, filters are used in a number of digital communication applications, such as equalization, echo cancellation, band selection, and the like. For example, a Digital Subscriber Line (DSL) system, such as an Asymmetric Digital Subscriber Line (ADSL) system, uses filters to process and provide specific attributes to the signals, which are transmitted over communications channels. More specifically, a Discrete Multitone (DMT) modulation transmission method can be used in ADSL. The DMT transmission divides the channel into several independent sub-channels making it easy to transmit data on each sub-channel. It is known in the art that a channel refers both to the physical channel and the mathematical representation of the channel (e.g., the channel impulse response). The overall data rate of the channel is the sum of the data rates over all these sub-channels. In this way, instead of transmitting data over one wideband channel, data is transmitted over the narrower sub-channels.
The transmission of successive DMT symbols (or packet of data) may allow inter-symbol interference (ISI) to appear due to the dispersive nature of the channel. One way to reduce or ideally eliminate ISI is to equalize the overall channel, for example, by appending a filter at the receiver end and making the overall channel impulse response at the receiver end very close or equal to a Dirac impulse. However, such a method is not efficient for the type of channels encountered in ADSL applications. For example, ADSL channels are dispersive; yielding a long impulse response, which implies an ISI corrupted signal together with a complex equalization structure. As ISI becomes more severe, the equalizer complexity rises rapidly and the computational cost increases to an unacceptable level. This implies that to keep a reasonable performance, a more complex, expensive, and powerful receiver must be adopted.
Another way to reduce or eliminate ISI is to insert between any two adjacent information symbols (such as DMT symbols) a time interval during which some non-information carrying data is transmitted. For example, a predefined data sequence may be transmitted. In any case, if this time interval (also known as guard period) is at least as long as the channel memory, the effects of ISI can be substantially isolated from one information symbol to the next information symbol, and processing may be performed in an information symbol by information symbol basis. If the transmitted information symbol is repeated, then the guard period is commonly referred to as the cyclic prefix because the repetition is essentially a cyclic extension of the information symbol. However, the channels used in ADSL applications, for example, have a long impulse response, which makes the guard period a waste of the available bandwidth.
A more practical solution combines the above two methods by both equalizing the channel and appending each information symbol with a guard period. This equalization method attempts to reduce the channel at the receiver end (i.e., the effective channel) to CP+1 taps, where CP is the cyclic prefix and taps are the coefficients of a filter. Accordingly, the channel may be coupled with a filter (e.g., a TEQ filter) to effectively shorten the channel to CP+1 significant taps, where significant taps are coefficient values that exhibit a significant or much higher value as compared to the value of the remaining coefficient values. By applying this method, ISI may theoretically be eliminated while maintaining a small overhead per transmitted information symbol (equal to the value of the CP).
Another concern in communications systems is noise. For instance, noise may be added to the analog signal while travelling over the channel (e.g., white Gaussian noise) by the process of digitizing the analog signal (e.g., quantization noise), and by the digital processing applied to the digitized signal. Those of skill in the art recognize that different kinds of noise will affect the signal quality in different ways. Furthermore, residual ISI usually has a flat spectrum and may contaminate the performance of the TEQ filter, which may decrease the ratio of the signal to noise power (SNR). A decrease in the SNR also reduces the data rate of the system, which is highly undesirable. As such, designing a TEQ filter that merely shortens the effective channel to CP+1 significant taps may also degrade the data rate, if the frequency response of the TEQ filter results in a significant attenuation of signal frequencies not originally attenuated. If the TEQ filter has a relatively flat frequency response (e.g., few ripples), then most useful or xe2x80x9cgoodxe2x80x9d frequencies will not be further attenuated. Information or data lost before the TEQ filter processing will remain lost, but substantially no additional losses will occur due to the TEQ filter processing.
Communications systems are often characterized by a transmitter side and a receiver side that communicate via a channel. FIG. 1 illustrates a communications system 101 having a transmitter 103, a channel 105, and a receiver 107. Transmitter 103 transmits data in the form of one or more information symbols (e.g., DMT symbols) across channel 105 to receiver 107. Such information symbols have a cyclic prefix appended at or attached to the beginning of each information symbol transmitted. As such, each information symbol is preceded by its cyclic prefix and transmitted over channel 105. If channel 105 is longer than the cyclic prefix, then ISI may result at receiver 107 of communications system 101. In order to avoid such ISI, channel 105 may communicate with a time domain equalizer (TEQ) filter (not shown) to shorten the channel to a desired length.
FIG. 2 illustrates an exemplary communications system 201 having a TEQ filter 217. Communications system 201 includes a transmitter side and a receiver side, where a channel 211 provides a medium of communication between the two. On the transmitter side, communications system 201 includes a signal processor 203, an inverse fast Fourier transform (IFFT) engine 205, a parallel to serial converter 207, and a transmitter filter 209. Channel 211 may be any medium commonly used in communications systems. For example, in ADSL transmission, channel 211 may be a twisted pair of wires. On the receiver side, communications system 201 includes a receiver filter 213, a fast Fourier transform (FFT) engine 219, and a frequency domain equalizer (FEQ) 221.
Communications system 201 receives data 223 and outputs filtered data 225. Signal processor 203 processes data 223 and communicates the results to IFFT engine 205, and IFFT engine 205 produces time-domain data. A cyclic prefix (CP) is appended to each time-domain information symbol by an add CP means 206, and the modified information symbol communicated to parallel to serial converter 207. Parallel to serial converter 207 then converts the modified information symbol into a serial signal. The serial signal is communicated to transmitter filter 209 (e.g., a digital to analog conversion means), which transforms the serial signal into a continuous time signal.
Channel 211 provides a medium for transmitting the continuous time signal to receiver filter 213, which may perform some analog and digital filtering to the incoming signal. Receiver filter 213 digitizes the continuous time signal into a digital signal. TEQ filter 217 filters the digital signal in order to reduce and ideally cancel the ISI. The cyclic prefix is removed from each information symbol of the digital signal by a remove CP means 218 resulting in a modified digital signal. The modified digital signal is communicated to FFT engine 219 to process the modified digital signal before FEQ filter 221 inverts the effect of the channel in each transmitted information symbol and outputs data 225.
In an ideal environment, data 225 is the same as data 223. In addition, it is desirable to transmit the data at a high data rate. However, solely focusing on shortening a channel may sacrifice the desired increase in data rate because TEQ filter 217 may introduce spectral nulls to the digital signal. Since a TEQ filter could introduce spectral nulls, the data rate of the communications system may decrease. One way to achieve an increase in the data rate while shortening the channel is to design a TEQ filter that may adapt to different channels in order to shorten the channel as desired. Therefore, it is desirable to both shorten the channel and also control the spectral characteristics of the TEQ filter. Thus, a TEQ design and method for its use is needed which shortens the channel of a communications system without sacrificing the data rate.
In accordance with one embodiment of the present invention, a spectrally flat TEQ filter design reduces the error between the outputs of a TEQ filter and a target filter in order to increase the data rate of a communications system. Such an embodiment of the present invention shortens the channel while taking into account the shape of the TEQ filter frequency response. Since spectral flatness is at odds with shortening of the channel, it is desirable to have a balance between the two in order to lessen the effects of ISI noise on a signal while maintaining an adequate data rate. In accordance with one embodiment of the present invention, the TEQ filter design reduces the error between the output of the TEQ filter and the target filter by constraining the central tap of the TEQ filter to a non-zero real number while calculating the remaining tap values for the TEQ filter and the target filter. For example, reduction of the error between the TEQ filter and the target filter may be accomplished by using an adaptive filter identification algorithm. Thus, a spectrally flat TEQ filter is provided, which reduces the error between the TEQ filter and the target filter in order to increase the data rate of the communications system.